This investigation is concerned with the functional role of Sendai virus surface glycoproteins in bringing about the fusion of the virion envelope with the plasma membrane of target cells. Several hypotheses regarding the aggregation phase of fusion have been constructed involving the cell attachment glycoprotein, HN, of the virion envelope. It will be determined whether this protein effects clustering of receptor molecules in the target membrane and whether such clustering can create protein-free domains for fusion. Electron microscopic analysis of the interaction will be emphasized. The role of target membrane cholesterol will be explored in this phase of the investigation. The effects of HN on the surface charge of the target membrane will also be studied. Another set of hypotheses address the contact and membrane perturbation phases of fusion. The role of the fusion protein, F, of the virion envelope will be investigated in this phase of the project. It will be determined whether the hydrophobic amino terminus of the F1 fragment is inserted into the target membrane by fluorescence energy transfer. The role of cholesterol in the target membrane as a "spacer" molecule providing "sites" for the insertion of F will be explored. Other characteristics of the F1 fragment in perturbing of the target membrane will be examined by synthesizing a series of peptides that will simulate the amino acid sequence of the native molecule or alternatively, produce by site-specific mutation, changes in the native sequence to test the effects of amino acids proximal to the insertion sequence. The synthesized and the mutant peptides will be incorporated into vesicles and tested for the ability to cause fusion. Finally we will examine the conformational requirements of HN and F in the virion surface for effecting fusion.